(1) Field of the Invention
The present invention relates to an aqueous surface-treating agent used for surface treatment of reinforcing material, a reinforcing material surface-treated with said surface-treating agent, and a composite material comprising said reinforcing material.
(2) Description of the Prior Art
In producing an intended molded product from, for example, a thermoplastic resin, it has been conducted to obtain said product in the form of a composite material which is a mixture of said thermoplastic resin and a reinforcing material, in order to allow said molded product to have improved properties. Various materials are in use as such a reinforcing material, and typical examples thereof include a glass fiber, a carbon fiber and an aramid fiber.
The glass fiber is produced by extruding a molten glass through a nozzle at a high speed, followed by solidification and winding. The glass fiber has low resistance particularly to friction and, when damaged at the surface, is broken easily. Hence, in order to prevent the damage of glass fiber yarn by contact with winder during winding after spinning or to prevent the entanglement, end breakage, raveling, fluffing, etc. of glass fiber yarn in loom, the glass fiber yarn after melt spinning is treated with a collecting agent for glass fiber to protect the fiber surface and then is wound.
As the collecting agent for glass fiber, there are used those of starch (e.g. gelatine) type or of resin emulsion type (the resin includes, for example, a polyvinyl acetate, an ethylene-vinyl acetate copolymer resin, a polyester resin, an epoxy resin, a polyacrylic acid resin and a polyurethane resin). These collecting agents for glass fiber are used during melt spinning, in the form of an aqueous system, an organic solvent system or a mixed system thereof.
In producing a composite material by reinforcing a composite material matrix with a glass fiber produced as above, the glass fiber is treated with a coupling agent (which is a compound capable of reacting with both the glass fiber and the composite material matrix) in order to enhance the adhesivity between the composite material matrix and the glass fiber. In this case, the glass fiber which has been treated with a collecting agent for glass fiber and whose surface has been coated and protected, has lower reactivity with the coupling agent owing to said treatment; therefore, it is necessary to remove the collecting agent prior to the reaction of the glass fiber with the coupling agent and enhance the reactivity of the coupling agent with the glass fiber surface.
The removal of the collecting agent from the glass fiber is generally conducted by a so-called heat-cleaning method which comprises allowing the glass fiber to stand at 400-600.degree. C. for ten-odd hours. Consequently, the glass fiber after removal of collecting agent has a significantly low strength as compared with the glass fiber before the treatment. Therefore, the glass fiber of significantly low strength after removal of collecting agent, even after having been treated with the coupling agent, still has insufficient properties as a glass fiber used for reinforcing a composite material matrix.
A carbon fiber has a high elastic modulus and is very lightweight and, when used, for example, as a reinforcing material for thermoplastic resin, can improve the properties (e.g. mechanical strengths) of the resin and can provide an excellent material for automobile parts, mechanical parts, etc. For this carbon fiber, an epoxy resin has been used as a surface-treating agent. This epoxy resin becomes an effective surface-modifying agent for carbon fiber when the resulting carbon fiber is added to a thermosetting resin which is a composite material matrix, such as epoxy resin or phenolic resin; however, when the composite material matrix is a thermoplastic resin, the epoxy resin as surface modifier for carbon fiber often has insufficient adhesivity to the matrix and fails to sufficiently improve the mechanical strengths of the matrix.
As the surface-treating agents for carbon fiber, other than the above-mentioned epoxy resin, there were proposed a polyamide resin (Japanese Patent Application Kokai No. 106752/1978), a polyether resin (Japanese Patent Application Kokai No. 299580/1987) and a polyimide resin (Japanese Patent Application Kokai No. 40569/1989). These resins, however, are insufficient in adhesivity to the composite material matrixes used and fail to exhibit sufficient effects. There was also proposed effective surface modification by the use of a polycarbodiimide resin (Japanese Patent Application Kokai No. 311069/1993). In this approach, however, there is used, as a surface treating agent, a resin solution obtained from the synthesis using an aprotic organic solvent (e.g. Toluene); therefore, there are problems, for example, the organic solvent gives a bad effect on human health, there is a fire hazard, and the solution has a short life for stable storage.
An aramid as a heat-resistant polymer, which is a wholly aromatic polyamide wherein all of the bivalent hydrocarbon groups bonded via an amide linkage are aromatic ring groups, when made into molded materials such as fiber and the like, has a high strength, a high elastic modulus and excellent heat resistance and therefore is useful in various applications. In particular, an aramid fiber, having a high strength and a high elastic modulus, is added as a reinforcing material to various thermoplastic resins, rubbers and thermosetting resins, and the resulting compositions are under evaluation for mechanical properties, heat resistance, etc.
According to the conventional technique, however, a combination of an aramid fiber and a matrix is unable to provide a composite material with good mechanical properties because they have inferior adhesivity to each other. Moreover, the aramid, whose amide group has an ability of forming a strong hydrogen bond, has high affinity between molecules; therefore, when the aramid is mixed with a composite material matrix, it is difficult to obtain a composition in which the aramid fiber is dispersed uniformly in the matrix.
In order to eliminate the above drawbacks of the aramid, the following proposals were made; however, no sufficient adhesivity has been obtained yet between the aramid and the matrix.
(1) Hydrolyze the surface of an aramid resin with an acid or a base and then introduce an amino group.
(2) Add an epoxy compound to the surface of an aramid fiber and then react the resulting aramid fiber with a matrix resin used.
(3) Form, on the surface of an aramid fiber, a chemical structure similar to that of a matrix resin used.
(4) Subject the surface of an aramid fiber to a plasma treatment.
(5) Use a coupling agent.